The present invention relates to video recorders of the type which provide a relatively limited recording capacity and which are capable of replaying the recorded video information either in real time or in a variable speed, slow motion replay mode. Such recorders are particularly adapted for use in televising or recording sporting events where a continuous video recorder of a substantially greater capacity may be used to provide a recording of the entire event. A recorder of the type to which the present invention is directed is selectively employed to record portions of the event which are later replayed, either at normal speed or in a manner to produce a slow motion effect.
The standard NTSC color video signal used in the United States consists of a succession of video frames, each frame consisting of two video interlaced fields each of which consists of a series of horizontal lines of video information, separated by horizontal line timing pulses. Each frame contains a field of a first type, termed an odd field, and a field of a second type, termed an even field. In order to produce the desired interlace between the two fields of a frame, the beginning of each even field occurs at a time offset by one half video line time with respect to the horizontal timing pulses, while the beginning of each odd field occurs with no offset.
Color video signals include a chroma component. The phase of the chroma component at the end of each field will lag the phase of the chroma at the beginning of the field by 90.degree.. Thus, it is seen there are frames of video information of a first type in which the chroma component varies in phase from 0.degree. to 180.degree. and frames of a second type in which the chroma component varies from 180.degree. to 360.degree.. In order for successful operation of a recorder, the recorder must provide during replay successive frames which alternate in frame type, with each of the frames containing a field of a first field type and a field of a second field type.
A typical prior art slow motion recorder is shown in U.S. Pat. No. 3,637,928, issued Jan. 25, 1972, to Poulett. The Poulett recorder uses four video recording disc surfaces with four corresponding record/playback heads to record, respectively, the four fields making up the frames of the first and second frame types. Each of the recording disc surfaces is rotated at the field rate and, during recording and playback, the record/playback heads are moved to predetermined recording tracks on the disc surfaces. During playback, the sequence in which the fields are replayed may be varied in order to produce various slow motion effects.
Another recording device is shown in publication No. B 347,661, published Mar. 16, 1976, under the Second Trial Voluntary Protest Program, with Iyama, et al as inventors. The Iyama, et al recorder records only single fields and reproduces them in such a manner that they are interlaced into frames for display. A major problem with respect to video recorders operating at 3600 r.p.m. (the field rate in the NTSC system) is the limitation which this imposes on the recorder storage capacity, since only one field can be stored in each recording track. Another problem with such disc recorders is that excessive wear of the disc and transducer heads may occur over a period of time.
U.S. Pat. No. 3,518,366, issued June 30, 1970, to Phan, discloses a video recording system capable of reproducing video information in a slow motion format. The Phan system uses a single recording disc which is rotated at the frame rate (1800 r.p.m.) of the video signal. A plurality of frames of video information are recorded on one side of the disc in a spiral recording track. A pair of record/replay transducers cooperate with a single circular recording track on the opposite side of the disc to construct a single frame from a field which is replayed from the spiral recording track. This synthesized frame, having identical fields, is replayed a number of times under control of a slow motion timer in order to produce a slow motion video output signal. The Phan system is somewhat limited in flexibility, however, since it is capable of providing slow motion reproduction rates only at integer multiples of the rate at which the information is recorded.
U.S. Pat. No. 4,058,840, issued Nov. 15, 1977, to Kasprzak, and assigned to the assignee of the present invention, discloses a video frame recorder which includes control circuitry to re-record a field from one-half of a disc recording track onto the other half of the recording track such that an interlaced frame, consisting of two identical fields, is produced for replay. Although using a disc frame recorder, the Kasprzak system is not capable of providing slow motion video signals during replay.
In video recorders of the type to which the present invention is directed, it is advantageous that the operator of the recorder have provided to him a visual indication of the recording operation and, during replay, a visual indication of the replay operation. A recorder of the type to which the present invention is directed permits short portions of a video signal to be recorded and then replayed, with or without an altered time base effect as desired by the recorder operator. This type of recorder typically has a relatively short real time storage capacity and thus video information is continuously recorded over previously recorded video information. When used to provide slow motion instant replay of a sporting event, for example, a recorder of this type will typically be left in the record mode. The operator may note an event of interest to which he may later wish to return. In the past, a recording indicator has typically been provided in the form of a dial arrangement which rotates as the transducer heads are stepped to successive recording tracks. The operator will know that if the event of interest occurred while the dial pointer was directed to a certain point on the dial, he may replay the event of interest by operating the recorder during replay such that the dial pointer again is directed to this point on the dial. With such an arrangement, however, it is necessary for the operator to view the indicator dial and, simultaneously, the video monitor. The operator must take care, as well, that the recorder not be left in the record mode for a period sufficient to result in new video information being recorded over the video information showing the event of interest. Therefore, the operator must watch the dial pointer closely and may be distracted and miss an event which he would otherwise prefer to record.